The erection of above-ground structures, particularly in relatively sandy soil, often requires the formation of in-ground retaining walls for use as a load-bearing foundation or as a barrier to prevent the collapse of soil into the excavated area. Where such excavations are made adjacent an existing structure, the retaining wall along the excavation line adjacent the existing structure is necessary to resist soil pressures established beneath the adjoining structure. If no retaining wall is formed, the soil beneath the adjoining structure can collapse outwardly into the excavation site and/or damage the existing structure. In addition to retention of soil, retaining walls of this type are often constructed to block the flow of ground water into the excavated area.
A number of methods have been used to form retaining walls around an excavation site where adjacent structures adjoin such a site. One method has been to employ piles formed of wood or steel which are driven along the excavation line to form the retaining wall. Alternatively, a row of bored holes are formed along the excavation line which receive reinforced concrete piles to form the retaining wall. Both of these methods produce retaining walls which are not water-tight and which may require substantial horizontal strengthening to maintain the desired alignment along the excavation line.
Trenching apparatus such as disclosed in U.S. Pat. No. 4,843,742 to Camilleri have been proposed as an alternative to the methods and apparatus of forming retaining walls mentioned above. In trenching apparatus of this type, a supporting base capable of being moved in a longitudinal direction along a trench line carries an elongated digger arm supported on one side thereof by a boom structure. The support base is drivingly connected to skids or track assemblies of the type employed in conventional bulldozers or other excavation equipment which are effective to move the support base and digger arm along the trench line to form a trench of the desired depth. Concrete is poured into the trench in which appropriate reinforcement bars have been inserted immediately behind the moving trenching apparatus. In this manner, an essentially continuously formed, reinforced concrete retaining wall is provided a the excavation site.
Located at the lower end of the digger arm of the trenching apparatus disclosed in the 4,843,742 Camilleri patent is a digging and propulsion unit. This unit is initially placed atop the ground along the trench line and is operated to dig downwardly with the digger arm into the soil to the desired depth of the trench. As the boom structure mounted to the support base moves the upper end of the digger arm along the trench line, the digging and propulsion unit located below ground propels the lower end of the digger arm within the trench by operation of digger teeth carried by drive chains.
One problem with the digging and propulsion units of the type described in U.S. Pat. No. 4,843,742 is the loss of tension in the chains thereof as a result of wear. As the chains loosen, they do not effectively propel the lower end of the digging arm. To restore the proper tension to the chains associated with the digging and propulsion unit, the digger arm must be withdrawn from the trench, the worn chain(s) removed, and new chains installed. Such a procedure is inefficient an time-consuming.
A second problem with digging and propulsion units of the type disclosed in U.S. Pat. No. 4,843,742 is the accumulation of digging debris on the sprockets and shafts within the digging and propulsion unit which carry and drive the chains. Dirt, clay, concrete, sand, rocks and other materials tend to adhere to the hubs of the sprockets within the digging and propulsion unit and interfere with the travel of the digging chain about the sprocket and/or rotation of the sprocket. Periodically, the digger arm must be lifted out of the trench and workmen must use implements to remove the debris from the sprockets, shafts and chains. This procedure is labor intensive, time-consuming and adds expense to the cost of erecting the retaining wall.
A third problem with digging and propulsion units of the type disclosed in U.S. Pat. No. 4,843,742 is the rapid wear of the teeth or tips mounted to the chain(s) which dig downwardly to the initial trench depth, and then propel the lower end of the digging arm. Particularly in rocky or compacted soil, such as clay, these tips tend to wear after a short period of time requiring replacement of the individual tips or the entire chain.